1. Field of the Invention
The present invention relates to a horizontal, front loading vacuum heat treating furnace that is equipped to nitride or case harden materials by the addition of nitriding gases during the heat cycle, and to rapidly cool the hardened materials by external gas cooling at positive pressures in a single chamber of the furnace.
2. Description of the Prior Art
Typical nitriding furnaces presently in use are pit type furnaces or, in some cases, horizontal furnaces containing an inconel or other steel alloy retort which holds the workload during the heat treatment cycle. Over time inconel and other steel alloy retorts will dissociate the ammonia, resulting in the creation of surface nitrides and altering the desired nitriding potential of the process. Inability to accurately maintain a constant nitriding potential leads to poor quality nitrided parts. The present invention does not utilize such a steel alloy retort or refractory chamber. The vacuum nitriding furnace according to the present invention utilizes all graphite internal parts in the hot zone which are inert to the nitriding and corrosive nature of the preferred processing gas-anhydrous ammonia. The absence of reactive alloys in the furnace retort chamber results in the workload being the only source for ammonia dissociation and provides the nascent nitrogen required to produce the nitrided case in the workload material.
While the present furnace is capable of maintaining vacuum pressures as low as 1×10−2 torr, it is designed to maintain a slightly positive pressure during the nitriding cycle and includes new and improved mechanisms to ensure even heating and uniform gas flow throughout the process. The furnace is also designed with the capability to rapidly cool the workload at atmospheric pressure in the same furnace chamber.
In typical prior art vacuum furnaces, such as disclosed in EPO 754768, a single chamber vacuum furnace is described as being formed on the interior as a chamber within a chamber. A single internal circulation fan is located on the furnace door within an outer chamber for circulating the cooling gas. Actuated gas delivery units contain a series of flapper nozzles that open to allow gas to flow into the interior chamber through closeable openings, and then close as the pressure builds. This structural design and the method described allow the introduction of cooling gas closer to the top of the workload. As the cooling gas becomes stagnant, the lower portals, which are closed during the heating cycle, are opened to allow the hot gas to exit into the gas recirculation chamber to be cooled and recirculated. There is no mention of the materials used in the heating chamber, nor is there any recognition of the unique problems associated with gas nitriding of materials.
Another example of a vacuum furnace having a convection heating system is described in U.S. Pat. No. 6,756,566. The furnace includes a hot zone and a plurality of gas injection nozzles for injecting a cooling gas into the heat treatment zone of the furnace. Each gas injection nozzle includes a flapper, or gas exit port, having a nozzle designed to allow inward flow of gas during cooling, but to impede outward flow during the heating cycle. The furnace has an outer chamber and an inner chamber within the outer chamber. The inner chamber hot zone enclosure is lined internally with a refractory material to resist the intense processing heat.
Both designs described in these prior art patents are subject to potential gas leakage during the heating cycle due to their inability to maintain a completely positive seal. Thus both designs can cause thermal gradients within the hot zone during processing and can result in non-uniform core hardness of the workload. Neither design includes the unique graphite baffling arrangement in the hot zone, as disclosed in the present invention, resulting in uniform core hardness of the workload.